My realization that the terrorists had won came in absurd circumstances.

I stood in a crowded Manhattan park just before Halloween, surrounded by cossetted dogs dressed as ballerinas, superheroes, soldiers, bunches of grapes. Cooing humans massed to snap pictures.

Nobody in this cheerful throng seemed to mind the crush. But I panicked, unable to shake the notion that we were a massive target. I had to get out. I left my blissed-out son with his father (though I didn’t want to) and inched my way out onto the street.

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There it dawned on me: I hadn’t been in a large crowd for years.

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Whenever there is a mass killing, we’re always told to go about our lives as usual. Keep traveling, shopping, enjoying concerts and civic celebrations. If we change our way of life, the thinking goes, the terrorists win.

Well, by that measure, they’ve beaten me pretty good.

I think about mass killings — by terrorists foreign and domestic, born of ideology or insanity or both — almost every day. I don’t go to big concerts anymore and the attack on the Bataclan in Paris has made me leery of smaller shows, too. At the office, I note the distance from my desk to the nearest exit (too far), and guess how long it would take an armed, disgruntled nihilist to get to my corner of the newsroom. At the movies, I watch for people who look like they might be planning mayhem. I scope out escape routes and hiding spots in stores and restaurants.

It’s naive, of course, to make these calculations, and not a little embarrassing. It’s also futile. Not just because the likelihood of an attack is still quite small, which it is, despite their relentlessness and growing frequency. But also because there’s really nothing you can do to fully protect yourself.

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It can happen anywhere — at giant public events like the Boston Marathon, but also at holiday parties held in suburban function rooms, like in San Bernardino
; at cafes or clinics; in schools or supermarkets. The randomness just fuels the fear.

I know I’m not the only one looking over my shoulder. Over the past few days, I’ve had many conversations with friends, rattling off the ways in which we’ve rearranged our lives to take account of the grim possibilities.

Those who study these things find that fear spikes no matter where terrorist attacks happen, and remains elevated long afterward, making us do things collectively we otherwise mightn’t: Avoid flying, grow less tolerant, support security measures that fly in the face of what America is about.

“Fear in and of itself can have significant effects, in terms of one’s health and how one lives one’s life, on the national economy, and how government policies get made,” said Samuel Justin Sinclair, a clinical psychologist at Mass. General who is an expert on responses to terror attacks.

In the aftermath of Wednesday’s massacre of 14 in California — the highest mass shooting toll in the United States since Sandy Hook, which was supposed to change everything — there’s been talk of how people have grown immune to the horror.

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For some of us, it’s not so much immunity as despair. There is no collective will to do much of anything that might make these massacres less likely. No matter how many people are mowed down, the guns still flood across the land, sped along by bullies draped in flags. The same so-called leaders who quiver at the thought of accepting Syrian refugees won’t even vote to keep assault weapons from those on the terrorist watch list. The gun lobby owns them so completely that no horror, no body count, is big enough to move them.

Yes, we’re on our own. And so we make our calculations, avoiding crowds, plotting escape routes, viewing strangers with suspicion.

And standing outside a crowded, joy-filled park, worrying and waiting for a child who doesn’t yet know the fear his mother can’t shake.